Rework reusability control in cancellable continuation (Kotlin#2581)

* Rework reusability control in cancellable continuation

    * Update initCancellability documentation and implementation to be aligned with current invariants
    * Make parentHandle non-volatile and ensure there are no races around it
    * Establish new reusability invariants
      - Reusable continuation can be used _only_ if it states is not REUSABLE_CLAIMED
      - If it is, spin-loop and wait for release
      - Now the parent is attached to reusable continuation only if it was suspended at least once. Otherwise, the state machine can return via fast-path and no one will be able to release intercepted continuation (-> detach from parent)
      - It implies that the parent is attached after trySuspend call and can be concurrently reused, this is where new invariant comes into play
    * Leverage the fact that it's non-atomic and do not check it for cancellation prematurely. It increases the performance of fast-path, but potentially affects rare cancellation cases

Fixes Kotlin#2564

kotlinx-coroutines-core/common/src/CancellableContinuation.kt

@@ -104,8 +104,10 @@ public interface CancellableContinuation<in T> : Continuation<T> {
     public fun completeResume(token: Any)
 
     /**
-     * Legacy function that turned on cancellation behavior in [suspendCancellableCoroutine] before kotlinx.coroutines 1.1.0.
-     * This function does nothing and is left only for binary compatibility with old compiled code.
+     * Internal function that setups cancellation behavior in [suspendCancellableCoroutine].
+     * It's illegal to call this function in any non-`kotlinx.coroutines` code and
+     * such calls lead to undefined behaviour.
+     * Exposed in our ABI since 1.0.0 withing `suspendCancellableCoroutine` body.
      *
      * @suppress **This is unstable API and it is subject to change.**
      */
@@ -332,7 +334,7 @@ internal suspend inline fun <T> suspendCancellableCoroutineReusable(
 internal fun <T> getOrCreateCancellableContinuation(delegate: Continuation<T>): CancellableContinuationImpl<T> {
     // If used outside of our dispatcher
     if (delegate !is DispatchedContinuation<T>) {
-        return CancellableContinuationImpl(delegate, MODE_CANCELLABLE_REUSABLE)
+        return CancellableContinuationImpl(delegate, MODE_CANCELLABLE)
     }
     /*
      * Attempt to claim reusable instance.

kotlinx-coroutines-core/common/src/CancellableContinuationImpl.kt

@@ -72,10 +72,7 @@ internal open class CancellableContinuationImpl<in T>(
      */
     private val _state = atomic<Any?>(Active)
 
-    private val _parentHandle = atomic<DisposableHandle?>(null)
-    private var parentHandle: DisposableHandle?
-        get() = _parentHandle.value
-        set(value) { _parentHandle.value = value }
+    private var parentHandle: DisposableHandle? = null
 
     internal val state: Any? get() = _state.value
 
@@ -93,7 +90,21 @@ internal open class CancellableContinuationImpl<in T>(
     }
 
     public override fun initCancellability() {
-        setupCancellation()
+        /*
+        * Invariant: at the moment of invocation, `this` has not yet
+        * leaked to user code and no one is able to invoke `resume` or `cancel`
+        * on it yet. Also, this function is not invoked for reusable continuations.
+        */
+        val handle = installParentHandle()
+            ?: return // fast path -- don't do anything without parent
+        // now check our state _after_ registering, could have completed while we were registering,
+        // but only if parent was cancelled. Parent could be in a "cancelling" state for a while,
+        // so we are helping it and cleaning the node ourselves
+        if (isCompleted) {
+            // Can be invoked concurrently in 'parentCancelled', no problems here
+            handle.dispose()
+            parentHandle = NonDisposableHandle
+        }
     }
 
     private fun isReusable(): Boolean = delegate is DispatchedContinuation<*> && delegate.isReusable(this)
@@ -118,40 +129,6 @@ internal open class CancellableContinuationImpl<in T>(
         return true
     }
 
-    /**
-     * Setups parent cancellation and checks for postponed cancellation in the case of reusable continuations.
-     * It is only invoked from an internal [getResult] function for reusable continuations
-     * and from [suspendCancellableCoroutine] to establish a cancellation before registering CC anywhere.
-     */
-    private fun setupCancellation() {
-        if (checkCompleted()) return
-        if (parentHandle !== null) return // fast path 2 -- was already initialized
-        val parent = delegate.context[Job] ?: return // fast path 3 -- don't do anything without parent
-        val handle = parent.invokeOnCompletion(
-            onCancelling = true,
-            handler = ChildContinuation(this).asHandler
-        )
-        parentHandle = handle
-        // now check our state _after_ registering (could have completed while we were registering)
-        // Also note that we do not dispose parent for reusable continuations, dispatcher will do that for us
-        if (isCompleted && !isReusable()) {
-            handle.dispose() // it is Ok to call dispose twice -- here and in disposeParentHandle
-            parentHandle = NonDisposableHandle // release it just in case, to aid GC
-        }
-    }
-
-    private fun checkCompleted(): Boolean {
-        val completed = isCompleted
-        if (!resumeMode.isReusableMode) return completed // Do not check postponed cancellation for non-reusable continuations
-        val dispatched = delegate as? DispatchedContinuation<*> ?: return completed
-        val cause = dispatched.checkPostponedCancellation(this) ?: return completed
-        if (!completed) {
-            // Note: this cancel may fail if one more concurrent cancel is currently being invoked
-            cancel(cause)
-        }
-        return true
-    }
-
     public override val callerFrame: CoroutineStackFrame?
         get() = delegate as? CoroutineStackFrame
 
@@ -188,7 +165,9 @@ internal open class CancellableContinuationImpl<in T>(
      */
     private fun cancelLater(cause: Throwable): Boolean {
         if (!resumeMode.isReusableMode) return false
-        val dispatched = (delegate as? DispatchedContinuation<*>) ?: return false
+        // Ensure that we are postponing cancellation to the right instance
+        if (!isReusable()) return false
+        val dispatched = delegate as DispatchedContinuation<*>
         return dispatched.postponeCancellation(cause)
     }
 
@@ -216,7 +195,7 @@ internal open class CancellableContinuationImpl<in T>(
 
     private inline fun callCancelHandlerSafely(block: () -> Unit) {
         try {
-           block()
+            block()
         } catch (ex: Throwable) {
             // Handler should never fail, if it does -- it is an unhandled exception
             handleCoroutineException(
@@ -276,9 +255,33 @@ internal open class CancellableContinuationImpl<in T>(
 
     @PublishedApi
     internal fun getResult(): Any? {
-        setupCancellation()
-        if (trySuspend()) return COROUTINE_SUSPENDED
+        val isReusable = isReusable()
+        // trySuspend may fail either if 'block' has resumed/cancelled a continuation
+        // or we got async cancellation from parent.
+        if (trySuspend()) {
+            /*
+             * We were neither resumed nor cancelled, time to suspend.
+             * But first we have to install parent cancellation handle (if we didn't yet),
+             * so CC could be properly resumed on parent cancellation.
+             */
+            if (parentHandle == null) {
+                installParentHandle()
+            }
+            /*
+             * Release the continuation after installing the handle (if needed).
+             * If we were successful, then do nothing, it's ok to reuse the instance now.
+             * Otherwise, dispose the handle by ourselves.
+            */
+            if (isReusable) {
+                releaseClaimedReusableContinuation()
+            }
+            return COROUTINE_SUSPENDED
+        }
         // otherwise, onCompletionInternal was already invoked & invoked tryResume, and the result is in the state
+        if (isReusable) {
+            // release claimed reusable continuation for the future reuse
+            releaseClaimedReusableContinuation()
+        }
         val state = this.state
         if (state is CompletedExceptionally) throw recoverStackTrace(state.cause, this)
         // if the parent job was already cancelled, then throw the corresponding cancellation exception
@@ -296,6 +299,28 @@ internal open class CancellableContinuationImpl<in T>(
         return getSuccessfulResult(state)
     }
 
+    private fun installParentHandle(): DisposableHandle? {
+        val parent = context[Job] ?: return null // don't do anything without a parent
+        // Install the handle
+        val handle = parent.invokeOnCompletion(
+            onCancelling = true,
+            handler = ChildContinuation(this).asHandler
+        )
+        parentHandle = handle
+        return handle
+    }
+
+    /**
+     * Tries to release reusable continuation. It can fail is there was an asynchronous cancellation,
+     * in which case it detaches from the parent and cancels this continuation.
+     */
+    private fun releaseClaimedReusableContinuation() {
+        // Cannot be casted if e.g. invoked from `installParentHandleReusable` for context without dispatchers, but with Job in it
+        val cancellationCause = (delegate as? DispatchedContinuation<*>)?.tryReleaseClaimedContinuation(this) ?: return
+        detachChild()
+        cancel(cancellationCause)
+    }
+
     override fun resumeWith(result: Result<T>) =
         resumeImpl(result.toState(this), resumeMode)
 
@@ -462,11 +487,10 @@ internal open class CancellableContinuationImpl<in T>(
 
     /**
      * Detaches from the parent.
-     * Invariant: used from [CoroutineDispatcher.releaseInterceptedContinuation] iff [isReusable] is `true`
      */
     internal fun detachChild() {
-        val handle = parentHandle
-        handle?.dispose()
+        val handle = parentHandle ?: return
+        handle.dispose()
         parentHandle = NonDisposableHandle
     }
 

kotlinx-coroutines-core/common/src/CoroutineDispatcher.kt

@@ -101,7 +101,12 @@ public abstract class CoroutineDispatcher :
 
     @InternalCoroutinesApi
     public override fun releaseInterceptedContinuation(continuation: Continuation<*>) {
-        (continuation as DispatchedContinuation<*>).reusableCancellableContinuation?.detachChild()
+        /*
+         * Unconditional cast is safe here: we only return DispatchedContinuation from `interceptContinuation`,
+         * any ClassCastException can only indicate compiler bug
+         */
+        val dispatched = continuation as DispatchedContinuation<*>
+        dispatched.release()
     }
 
     /**

kotlinx-coroutines-core/common/src/JobSupport.kt

@@ -1228,6 +1228,8 @@ public open class JobSupport constructor(active: Boolean) : Job, ChildJob, Paren
          * thrown and not a JobCancellationException.
          */
         val cont = AwaitContinuation(uCont.intercepted(), this)
+        // we are mimicking suspendCancellableCoroutine here and call initCancellability, too.
+        cont.initCancellability()
         cont.disposeOnCancellation(invokeOnCompletion(ResumeAwaitOnCompletion(cont).asHandler))
         cont.getResult()
     }

kotlinx-coroutines-core/common/src/internal/DispatchedContinuation.kt

@@ -46,17 +46,15 @@ internal class DispatchedContinuation<in T>(
      * 4) [Throwable] continuation was cancelled with this cause while being in [suspendCancellableCoroutineReusable],
      *    [CancellableContinuationImpl.getResult] will check for cancellation later.
      *
-     * [REUSABLE_CLAIMED] state is required to prevent the lost resume in the channel.
-     * AbstractChannel.receive method relies on the fact that the following pattern
+     * [REUSABLE_CLAIMED] state is required to prevent double-use of the reused continuation.
+     * In the `getResult`, we have the following code:
      * ```
-     * suspendCancellableCoroutineReusable { cont ->
-     *     val result = pollFastPath()
-     *     if (result != null) cont.resume(result)
+     * if (trySuspend()) {
+     *     // <- at this moment current continuation can be redispatched and claimed again.
+     *     attachChildToParent()
+     *     releaseClaimedContinuation()
      * }
      * ```
-     * always succeeds.
-     * To make it always successful, we actually postpone "reusable" cancellation
-     * to this phase and set cancellation only at the moment of instantiation.
      */
     private val _reusableCancellableContinuation = atomic<Any?>(null)
 
@@ -66,9 +64,9 @@ internal class DispatchedContinuation<in T>(
     public fun isReusable(requester: CancellableContinuationImpl<*>): Boolean {
         /*
          * Reusability control:
-         * `null` -> no reusability at all, false
+         * `null` -> no reusability at all, `false`
          * If current state is not CCI, then we are within `suspendCancellableCoroutineReusable`, true
-         * Else, if result is CCI === requester.
+         * Else, if result is CCI === requester, then it's our reusable continuation
          * Identity check my fail for the following pattern:
          * ```
          * loop:
@@ -82,6 +80,27 @@ internal class DispatchedContinuation<in T>(
         return true
     }
 
+
+    /**
+     * Awaits until previous call to `suspendCancellableCoroutineReusable` will
+     * stop mutating cached instance
+     */
+    public fun awaitReusability() {
+        _reusableCancellableContinuation.loop { it ->
+            if (it !== REUSABLE_CLAIMED) return
+        }
+    }
+
+    public fun release() {
+        /*
+         * Called from `releaseInterceptedContinuation`, can be concurrent with
+         * the code in `getResult` right after `trySuspend` returned `true`, so we have
+         * to wait for a release here.
+         */
+        awaitReusability()
+        reusableCancellableContinuation?.detachChild()
+    }
+
     /**
      * Claims the continuation for [suspendCancellableCoroutineReusable] block,
      * so all cancellations will be postponed.
@@ -103,11 +122,20 @@ internal class DispatchedContinuation<in T>(
                     _reusableCancellableContinuation.value = REUSABLE_CLAIMED
                     return null
                 }
+                // potentially competing with cancel
                 state is CancellableContinuationImpl<*> -> {
                     if (_reusableCancellableContinuation.compareAndSet(state, REUSABLE_CLAIMED)) {
                         return state as CancellableContinuationImpl<T>
                     }
                 }
+                state === REUSABLE_CLAIMED -> {
+                    // Do nothing, wait until reusable instance will be returned from
+                    // getResult() of a previous `suspendCancellableCoroutineReusable`
+                }
+                state is Throwable -> {
+                    // Also do nothing, Throwable can only indicate that the CC
+                    // is in REUSABLE_CLAIMED state, but with postponed cancellation
+                }
                 else -> error("Inconsistent state $state")
             }
         }
@@ -127,14 +155,13 @@ internal class DispatchedContinuation<in T>(
      *
      * See [CancellableContinuationImpl.getResult].
      */
-    fun checkPostponedCancellation(continuation: CancellableContinuation<*>): Throwable? {
+    fun tryReleaseClaimedContinuation(continuation: CancellableContinuation<*>): Throwable? {
         _reusableCancellableContinuation.loop { state ->
             // not when(state) to avoid Intrinsics.equals call
             when {
                 state === REUSABLE_CLAIMED -> {
                     if (_reusableCancellableContinuation.compareAndSet(REUSABLE_CLAIMED, continuation)) return null
                 }
-                state === null -> return null
                 state is Throwable -> {
                     require(_reusableCancellableContinuation.compareAndSet(state, null))
                     return state

kotlinx-coroutines-core/jvm/test/CancelledAwaitStressTest.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ * Copyright 2016-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
  */
 
 package kotlinx.coroutines
@@ -52,4 +52,4 @@ class CancelledAwaitStressTest : TestBase() {
     private fun keepMe(a: ByteArray) {
         // does nothing, makes sure the variable is kept in state-machine
     }
-}
+}

kotlinx-coroutines-core/jvm/test/FieldWalker.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016-2020 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ * Copyright 2016-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
  */
 
 package kotlinx.coroutines
@@ -56,7 +56,7 @@ object FieldWalker {
      * Reflectively starts to walk through object graph and map to all the reached object to their path
      * in from root. Use [showPath] do display a path if needed.
      */
-    private fun walkRefs(root: Any?, rootStatics: Boolean): Map<Any, Ref> {
+    private fun walkRefs(root: Any?, rootStatics: Boolean): IdentityHashMap<Any, Ref> {
         val visited = IdentityHashMap<Any, Ref>()
         if (root == null) return visited
         visited[root] = Ref.RootRef

kotlinx-coroutines-core/jvm/test/ReusableCancellableContinuationLeakStressTest.kt

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2016-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.coroutines
+
+import kotlinx.coroutines.channels.*
+import org.junit.Test
+import kotlin.test.*
+
+class ReusableCancellableContinuationLeakStressTest : TestBase() {
+
+    @Suppress("UnnecessaryVariable")
+    private suspend fun <T : Any> ReceiveChannel<T>.receiveBatch(): T {
+        val r = receive() // DO NOT MERGE LINES, otherwise TCE will kick in
+        return r
+    }
+
+    private val iterations = 100_000 * stressTestMultiplier
+
+    class Leak(val i: Int)
+
+    @Test // Simplified version of #2564
+    fun testReusableContinuationLeak() = runTest {
+        val channel = produce(capacity = 1) { // from the main thread
+            (0 until iterations).forEach {
+                send(Leak(it))
+            }
+        }
+
+        launch(Dispatchers.Default) {
+            repeat (iterations) {
+                val value = channel.receiveBatch()
+                assertEquals(it, value.i)
+            }
+            (channel as Job).join()
+
+            FieldWalker.assertReachableCount(0, coroutineContext.job, false) { it is Leak }
+        }
+    }
+}